Referring to FIG. 1, a conventional bicycle hub 10 is composed of a shell 12, a chain wheel 14, a plurality of pawls 16, and a plurality of tongues 18. The shell 12 includes an axial hole 122, an arbor 124 coaxially inserted into the axial hole 122, and a plurality of indentations 126 formed on an internal sidewall of the axial hole 122. The chain wheel 14 is sleeved onto the arbor 124, having an annular stub 142 engaging the axial hole 122 and a plurality of concavities 144 formed on an external periphery of the annular stub 142. Each of the pawls 16 is pivotably mounted to an end of one of the concavities 144 and stopped against one of the indentations 126. Each of the tongues is mounted to the other end of the concavity 144 and stopped against one of the pawls 16.
When a user treads pedals of a bicycle to drive the chain wheel 14 to rotate forward through a chain, each of the pawls 16 pushes against the indentations 126 to drive rotation of the shell 12, as shown in FIG. 2, whereby the bicycle wheel is rotated forward to drive the bicycle to move forward.
Bicycles are very efficient machines. They allow a rider to cover relatively large distances at a faster pace than walking, and a cyclist is enabled to perform the task with the expenditure of less energy overall. By comparison, cyclists have a relatively limited supply of energy when measured against a vehicle using an internal combustion engine, and because only the rider is capable of producing the energy needed to propel the bicycle, it is therefore desirable to keep the bicycle's efficiency at its absolute maximum.
Most bicycles are fitted with some form of over-running mechanism, such as a “freewheel” that will allow the rider to “coast” (stop pedaling) while the forward motion of the bicycle continues. Such as the one described above. However, all such existing mechanisms also introduce an additional amount of drag on the wheel and therefore they lessen the bicycle's overall efficiency.
These over-running mechanisms can be grouped into two broad categories. The first types are those that introduce drag when the rider is coasting. These designs are by far the most commonly used, with the traditional ratchet mechanism being the predominant style. When the cyclist applies power to a traditional ratchet, the mechanism does not add significant drag (above that within the bearings of the drive mechanism to the axle) because the cogged driver moves with the wheel, and, as a result there is no relative movement between the elements. However, when the rider chooses to “freewheel”, the cogged driver then becomes stationary as the wheel continues to rotate. This onward rotation of the wheel requires the ratchet pawls to be rubbing continually against the inside faces of a toothed engagement ring, and this action introduces sliding friction. Additionally, during freewheeling the ratchet pawls are forced to oscillate radially as each engagement ring “tooth” moves past because the ratchet pawls are being biased outwardly by their springs. In this example, the wheel will have to “give” energy to the pawl to accelerate it radially inwards and it must also “give” energy in order to compress the pawl springs. As a result, this energy is also “lost” as it is converted into heat and sound.
One may easily demonstrate these energy losses by turning a bicycle upside-down and spinning the front and rear wheels to simulate forward motion. The rear wheel will come to a halt far more quickly than the front wheel, which does not have to cope with these additional losses. These energy losses are not present when the rider is pedaling, but in both leisure and competition use there are many times in which a rider will choose to coast, such as when riding downhill or when “drafting” behind another cyclist in the racing “peloton” (a large group of cyclists traveling together). For example, it has been estimated that a bicycle racer participating in a stage of the Tour de France may “coast” for 20-25% of the time when traveling in the draft of the “peloton” during a typical stage that occurs in the first week of the event. In a sport where conserving energy is a critical component to achieving victory, it becomes clear that it is very desirable to create gains in efficiency wherever possible.
Hubs that employ a clutch rather than a ratchet mechanism are also available but are used with far less frequency. In these hubs, forward movement of the driver relative to the axle will cause a clutch element to move into frictional engagement with the hub of the wheel. Unlike with the ratchet mechanism, these hubs will disengage the driver from the wheel entirely when the rider back-pedals slightly, and as such these hubs will coast without additional friction from the drive mechanism. If a bicycle equipped with the clutch device is turned upside down, the front and back wheels will spin equally and freely when rotated to simulate forward motion. However, drag must remain present at all times when the rider is pedaling because these mechanisms rely on drag between the clutch mechanism and the axle in order to engage the wheel's hub. Due to drag being present, a significant amount of the rider's energy is constantly being lost to drag whenever pedaling occurs. For this reason, these hubs are generally only used on bicycles in which efficiency is not of paramount importance.
Thus, there is still a need for a bicycle drive mechanism in which it is possible for the wheel to over-run or coast with minimal to no energy being lost to the drive mechanism, and for the drive mechanism to run in an engaged state with no additional energy losses. More succinctly, the goal is to create a drive mechanism that is more efficient in its overall use than the existing designs that are available.